1. Field of the Invention
The present invention relates to an electroluminescence display device and a method of manufacturing the same, and, more particularly, to an electroluminescence display device which is provided with an improved substrate and thin-film layers, thereby achieving an improvement in light output efficiency depending on the refractive index of each thin film, and a method of manufacturing the same.
2. Description of the Related Art
Generally, electroluminescence display devices are high-level image display devices. They are flat display devices having many advantages such as a small size, a light weight, environmental resistance, durability, a long life span, and a wide viewing angle.
In electroluminescence display devices, when a voltage is applied to both ends of a fluorescent layer, electrons are accelerated toward the inside of the fluorescent screen so as to collide with an atom, i.e., a luminescence center. Thus, the electrons at the electron level of the atom are excited to a higher energy level, and then make a transition to a ground state. At this time, due to an energy gap around the electrons, light having a particular wavelength range is produced, that is, an electroluminescence phenomenon occurs.
Electroluminescence display devices are divided into alternating current (AC) drive types and direct current (DC) drive types, or thin-film types and thick-film types. Such electroluminescence display devices usually have at least one insulation layer and at least one fluorescent layer, and are provided with an electrode, which applies a voltage to both ends of the fluorescent layer. In order to improve the characteristics of the insulation layer and the fluorescent layer, these layers can be formed in a multilayer structure having a plurality of thin films made of different materials.
FIG. 1 shows an example of an AC drive type thin-film electroluminescence display device. Referring to FIG. 1, a first electrode layer 11, a first insulation layer 12, a fluorescent layer 13, a second insulation layer 14, and a second electrode layer 15 are sequentially formed on a substrate 10. The fluorescent layer 13 can be made of a metal sulfide such as ZnS, SrS, or CsS, an alkaline-earth potassium sulfide such as CaCa2S4 or SrCa2S4, or a metal oxide. For the atoms, i.e., luminescence centers, contained in a material of the fluorescent layer 13, transition metals containing Mn, Ce, Tb, Eu, Tm, Er, Pr, or Pb or alkaline ash metals can be used.
Another example of an electroluminescence display device is disclosed in Japanese Patent Publication No. 2001-176671. This electroluminescence display device has a structure in which a first electrode, an insulation layer of an inorganic compound, a luminescence layer of an inorganic compound, and a second electrode are stacked on a substrate.
In the electroluminescence display device shown in FIG. 1, when a predetermined voltage is applied to the first and second electrode layers 11 and 15 located at both sides of the fluorescent layer 13, light having a particular wavelength is produced due to an electroluminescence phenomenon.
In such an electroluminescence display device, a substantially flat thin film forms the top surface of each of the fluorescent layer 13 and the first and second electrode layers 11 and 15, and the refractive index of the first and second electrode layers 11 and 15 is high, so most of the light produced in the fluorescent layer 13 is not transmitted through the fluorescent layer 13 and the second electrode layer 15. Accordingly, only about 10% of the light is radiated from the electroluminescence display device.
More specifically, light efficiency of an electroluminescence display device is divided into internal efficiency and external efficiency. While the internal efficiency depends on the characteristics of the fluorescent layer, i.e., luminescence material, the external efficiency depends on the refractive index of each layer constituting the display device. The external efficiency ηex can be expressed as ηex=ηin×ηout. Here, ηin denotes the internal efficiency ηout denotes output coupling efficiency. A major restriction on the output coupling efficiency in a thin film electroluminescence display is related to the extraction of light generated inside the device to the external environment. The large mismatch between the refractive index of the thin film phosphor and air results in a large proportion of the light rays undergoing total internal reflection. Some of the light generated inside the thin film phosphor thus becomes trapped, unable to escape into the air. This effect plagues EL structures employing the thin film phosphor. According to Snell's law, only light emitted at an angle less than the critical angle can escape from the surface, all other light is internally reflected back into the device. A fluorescent layer made of a material such as ZnS usually has a high refractive index, and thus has a low output efficiency. The output efficiency depends on the formula ηe=(2n2)−1. Here, “n” denotes the refractive index of a fluorescent layer.
According to the above formula, in the case of a ZnS-based fluorescent layer, “n” is 2.5, so only about 8% of the light is output, while most of the light is guided between thin-film layers of the image display device and disappears.
In order to overcome the above problem, there has been proposed a method of adjusting the grain size of a fluorescent layer in an electroluminescence display device to provoke scattering on the surface of a substrate made of glass, thereby increasing the output of light. This method is effective when only a thin film fluorescent layer is formed on a substrate, but is, not significantly effective when an electrode layer and an insulation layer are formed between a fluorescent layer and a substrate.
In order to increase the light output efficiency of a fluorescent layer, a method of adjusting the partial pressure of O2 to 200 or greater mtorr during the formation of the fluorescent layer, so as to increase a nodule dimension to 100 nm, was proposed (S. I. Jones, D Kumar, K.-G. Cho, R. Singh, and P. H. Holloway, 1999, Displays, 19, 151), and a method for increasing a light output characteristic by four times using a rough glass substrate was proposed (Sella, C.; Martin, J.; Charreire, Y. 1982, Thin Solid Films, 90, 181).
Since light is prevented from being guided by only a particular thin-film layer, these methods have a limitation in increasing light output efficiency.
U.S. Pat. No. 6,476,550 discloses an organic electroluminescence display device including a thin film having a rugged surface for diffracting light.